Since we're all meant to have a doppelganger, it seems rational to suppose that we have an exact opposite too, and I've met the living antithesis of Jeremy Kyle. Louis Theroux is the very opposite of judgmental or preachy and his style – listening, re-evaluating his own prejudices, trusting his subjects and winning trust back – has served him well in documentaries on everything from our more colourful home characters (Jimmy Savile, the Hamiltons) to gang-life in the slums of Lagos and a jail for paedophiles.

His latest outing, exploring the tendency for parents in America to medicate scarily young children, seems at the start to invite viewers' jaws to drop. In Theroux's hands, however, you soon realise these snap judgments are often misguided. Sometimes the drugs do work.

"At some level, yes, I'd suspected things like ADHD were slightly suspicious labels, for lazy parents, but there's more to it," he tells me, in his thoughtful monotone. "And yes, we are moving to substituting a diagnostic language for the old moral language. But I don't know that that's altogether a bad thing. One thing that brought me to the story was having my own relatively new kids and realising that they are born with personalities, and so there can be such a thing as an easy kid and a difficult kid. Even with good parents, one child can present more challenges."

None of his conclusions are glib: often they are simply more questions. "When looking for a subject, I'm just intrigued by decisions that seem strange or counter-intuitive, and worlds in which I will feel alien but not so alien that there can be no journalistic purchase. There has to be some part for me to play, rather than just gawping.

"And if I've learned anything, in the older films and the more recent, it's that I'm constantly surprised by how humane instincts flourish in the darkest places, and also shocked by people's willingness to hurt one another. But there's nowhere absolutely without hope. Prisons, max-security mental hospitals. I try not to quote Nietzsche, but there was something about whenever you name something it tends towards its opposite. As soon as you define something as morally or ethically repulsive you start to become surprised by its positive values."